The answer lies within the quality of your three legal work experience placements. If all three placements were based in top law firms I would definitely recommend you put them all down. There should be some questions on the application form that will allow you to get some of the details of your commercial internship across. This way all of your experience will be covered. However, if one of your legal placements was at a high street firm then you might want to think about replacing it with your internship.